1. Field of the Invention
This invention relates generally to apparatus for automatically adjusting the focus of an objective lens used to inspect an article and more particularly, to an electro-optical system that utilizes fiber optics bundles, a multi-apertured projection reticle and a pair of multi-apertured return masks and corresponding detectors to derive control signals which are used to vary the physical separation between microscope objectives and an article or object to be inspected so as to bring the microscope into a position of focus relative thereto.
2. Description of the Prior Art
In apparatus for automatically inspecting articles such as photomasks and the like, it is important that means be included to automatically accomplish the focusing lf lenses used in the inspection process. One prior art apparatus providing for the automatic focusing of a lens for photomask inspection relies upon that fact that the magnification of an article viewed through a lens system is a function of the distance of the article from the image plane of the lens. The system relies upon the fact that as the article moves further away from the lens and its ideal focus position, a point in the periphery of the field moves inwardly toward the optical axis of the lens, and similarly, as the object gets closer to the lens a point in the periphery of the field moves outwardly and away from the optical axis. Thus, by focusing a narrow beam of light on a point in the periphery of the field and then detecting the position of this point relative to a known position, the spacing between the objective and the inspected article can be determined. Although this system works well where the surface being inspected is generally smooth, it does not provide acceptable accuracy for multi-level surfaces of the type present on silicon wafers having integrated circuits or the like formed therein.
An automatic lens focusing system designed for inspecting semiconductor wafers and the like is disclosed in the co-pending application of Paul Sandland et al, Ser. No. 582,584, filed Feb. 22, 1984 and assigned to the assignee of the present invention, and of which this application is a continuation-in-part. The description of such system is expressly incorporated herein by reference.